This is a real plot hole
After being served, Gittes goes to Hollis’s office and is told he’d been gone since lunch. Gittes then goes to Mulwray’s house, where Evelyn tells him Hollis will be back in the evening. We see the house is full of servants: the butler, the maid, the gardener, the man washing the car. He then goes to the reservoir where Hollis’s body has been discovered.
[spoiler]We find later that Hollis was drowned in the pond that afternoon. How was Hollis killed, and his body removed from the grounds, unobserved, when the estate was full of servants? Before someone says, “They must’ve bribed the servants,” we know at that very least that Khan, the ubiquitous butler/major domo, was loyal to Evelyn: he was hiding Katherine for her. It would’ve been impossible to keep a killing and body removal from him.
More salient than that: Towne explicitlyt accounted for every other incident in the story. But this aspect is completely unexplained. I’m tempted to write him and ask about it!